Carbon particles such as inorganic constituents, graphitic C-constituents as well as higher aliphatic, alicyclic and aromatic hydrocarbons, may occur during all technical combustion processes with carbon-carrying compounds.
Particularly serious problems occur in the case of diesel engine particle emissions because of the content of higher-molecular hydrocarbons as well as polycondensed aromatic compounds, the problems can range from being a health risk to possibly causing cancer, for example, from 3,4 benzpyrene and nitro aromatic compounds.
According to past experience, in the case of diesel combustion engines, engine-related measures are not sufficient for clearly reducing the emitted mass of black. These measures are impaired by the difficulty that, as a rule, a particle reduction by means of engine-related measures results in an increase of the NO.sub.x -emission (essentially NO), for example, by an increase of the relevant combustion chamber temperatures.
It is therefore necessary to achieve a particle reduction by means of measures taking place behind the engine. Mainly, attempts are made to combat the problem by installing particle filters into the exhaust gas duct.
Because of the black depositing process, the filter will become increasingly clogged over time which leads to a considerable increase of the exhaust backpressure (filter backpressure) and thus to a reduction of the engine output.
In the case of technical combustion processes, e.g. a combustion process of any natural fuel with atmospheric air, with a high air excess (for example, diesel engines), the oxygen free concentration in the exhaust gas will basically be sufficient for being able to cause the burning of the oxidizable black constituents. In this case, the absolute thermodynamic equilibria in the case of all exhaust gas conditions in question (black concentrations, partial oxygen pressure, overall pressure, temperature) are found in the direction of the quantitative oxidation to carbon dioxide and water. Because of reaction-kinetic criteria (activation energy for the ignition of black), in the case of black-loaded particle filters, the thermodynamically advantageous conditions are implemented without any additional measures starting only at approximately 600.degree. C. to a sufficiently rapid degree. As a rule, these temperatures are not available in diesel engine exhaust gases (and also in other technical combustions). One possibility for igniting the black on the particle filters consists of heating the filter, for example, by means of external burners, electrical resistance heating of the filter, and electromagnetic filter heating via high-frequency fields.
Currently, all of the above-mentioned methods are more or less in advanced development stages.
Another possibility exists in that the ignition temperature of filter-adsorbed black is either by means of correspondingly catalytic active substances (applied as a filter coating/filter impregnation) or by chemical promoters (additives in the fuel or separate injecting into the exhaust gas duct in front of the particle filter) lowered to such an extent that the normally utilizable exhaust gas temperatures, i.e., approximately 200.degree. C.-450.degree. C., will be sufficient for the complete burning-off of black.
So far, useful solutions based on particle filter systems have not been successfully implemented.
Because of the thus far largely negative experiences with filter systems (particularly concerning the special requirements in the case of diesel engines of passenger cars and commercial vehicles), the development of no-filter particle reduction processes is of particular importance.
In this context, a non-reactive process, developed by Robert Bosch GmbH of Stuttgart, Germany, is known, wherein carbon particles, after a preceding gas phase agglomeration (such as, an electrostatic charging) by means of a gasdynamic centrifugal precipitation, i.e., cyclone, are precipitated into a collecting vessel.
It is therefore an object of the present invention to provide a process for reducing carbon particles in exhaust gas flows.
An object of the invention consists of the direct oxidative conversion of carbon particles in the oxygenous medium of a free exhaust gas flow in a high-frequency-induced stationary plasma zone. Because of the presence of high-energy particles in the plasma (ions/electrons; electronically, vibronically and rotatorily excited ions or neutral molecules, activated carbon particles), the reaction-kinetic inhibitions (with respect to normal oxidation of carbon particles by means of molecular oxygen) are eliminated, i.e., there is extensive reduction of the activation energies. This results in a sequence of shock-controlled homogeneous gas reactions with high effective reaction rates.
With respect to the technical applicability of the process for exhaust gas systems, typical high-pressure plasmas (p&gt;1 bar) are required.
When plasma is produced by means of high-frequency fields, in the case of non-magnetic materials, the coupling-in of the high-frequency energy depends on the complex relative permittivity of the material: EQU .epsilon.=.epsilon.'+i.epsilon." (1)
or on the dielectric loss angle: EQU tan .delta.=.epsilon."/.epsilon.' (2)
wherein .epsilon. is a function of the temperature and of the frequency.
The volume-specific absorption of HF-energy in the interior of an HF-absorbing material is given by: EQU P.sub.abs =.pi..nu..epsilon.' tan .delta..vertline.E.vertline..sup.2( 3)
wherein .nu. is the frequency and E is the mean electric field intensity in the absorbing volume: EQU .epsilon..sub.o =8,859.multidot.10.sup.-12 Asec/Vm.
For matter whose losses are defined predominantly by the electric conducting capacity, the following applies: EQU .epsilon."=.sigma./2.pi..multidot..nu. (4)
wherein .sigma. is the electric conducting capacity in (.OMEGA.m).sup.-1. Therefore, the following is obtained for the convertible dissipated energy density: ##EQU1## The electromagnetic field which penetrates into an absorbing volume is weakened by absorption. As a result, depending on the matter and frequency of the electromagnetic field, a limited penetration depth d.sub.c is obtained: ##EQU2## wherein c=3.multidot.10.sup.8 m/sec, that is, the velocity of light.
When plasma is produced by high-frequency energy, a differentiation should be made between the process of plasma ignition and the process of maintaining a stationary plasma.
In the case of gases, the electrical conducting capacity is low so that comparatively high local field intensities are required for the plasma ignition or breakdown. In air, such breakdown field intensities are between 10-25 kV/cm.
However, as soon as such a plasma breakdown had been implemented, the relevant electromagnetic substance characteristics changed drastically (for example, the complex refractive index e mainly in the shape of the imaginary part and therefore according to (4) the conducting capacity .sigma.).
In particular, the conducting capacity .sigma. changes on the basis of the presence of free charge carriers by several powers of ten.
The electrical conducting capacity of a fully ionized plasma (full thermodynamic equilibrium (VTG) or local thermodynamic equilibrium (LTG) may be derived in a general form from Boltzmann's impact equation with the assumption of an ideal Lorentz gas, i.e., fully ionized gas with no electron interaction and stationary ions.
The following is obtained: ##EQU3## e.sub.o : electrical elemental charge; m.sub.el : electron mass;
N.sub.el : particle density electrons. PA1 Z.sub.i,a : system condition sums, PA1 E.sub.i : ionization energy. PA1 Electrostatic (Coulomb) forces PA1 Magnetic (Lorentz) forces PA1 Forces resulting from viscosities (Stokes forces) PA1 Forces resulting from pressure gradients. PA1 m=m.sub.el : electron cyclotron resonance PA1 m=m.sub.Ion : ion cyclotron resonance PA1 field concentration (E-vector) in rectangular wave guide systems, for example, R26 for 2.46 GHz-technology (H.sub.10 -geometry), i.e., standard technology. Additional field concentrations in the wave guide by capacitively operating stubs. PA1 excitation of electric or magnetic fundamental modes (such as E.sub.010, H.sub.111) or higher modes in cavity resonators (cylindrical).
In the case of VTG conditions and LTG conditions, the Saha-Eggert Equation applies to N.sub.el : ##EQU4## N.sub.i.sup.+ : particle density ions, N.sub.a : particle density neutral gas molecules,
The temperature-dependent electric conducting capacities of VTG plasmas and LTG plasmas are determined mainly by the impact cross-sections between free electrons. They are therefore proportional to the number density N.sub.el of the free electrons.
The presence of considerable concentrations of free charge carriers, after the plasma ignition has taken place, finally influences the further action of the stationary plasma with respect to the coupling-in (maintaining the plasma condition) of high-frequency power.
The question therefore arises concerning the propagation possibility of electromagnetic waves in a fully ionized plasma.
Many different types of electromagnetic waves may form in such media because of different characteristics of the electron gas and ion gas as well as the following acceleration-effective processes:
For the special case of an exclusive Coulomb interaction without a stationary magnetic field with the additional marginal conditions of vanishing damping, .sigma..fwdarw..infin., charge maintenance and quasi-neutrality in the plasma, the following applies to flat electromagnetic waves: ##EQU5## with the dispersion relation: EQU .omega..sub.p =(N.sub.el e.sub.o.sup.2 /(m.sub.el .epsilon..sub.o)).sup.1/2( 11)
.omega..sub.p is the characteristic plasma (Langmuir) frequency.
Accordingly, transverse electromagnetic waves can propagate in a stationary plasma only if .omega.&gt;.omega..sub.p. For .omega.&lt;.omega..sub.p, there is a cut-off because of total reflection of the incoming electromagnetic waves (no further HF-energy absorption), and the plasma condition will break down.